Chiral sensing with achiral isotropic metasurfaces

TL;DR

This paper provides a theoretical framework for using achiral isotropic metasurfaces to detect molecular chirality by analyzing magneto-electric coupling and circular dichroism signals, enabling unambiguous chirality determination.

Contribution

It introduces an analytical and numerical approach to understand chiroptical signals from achiral metasurfaces, highlighting the independent roles of the real and imaginary parts of the chirality parameter.

Findings

Enhanced magneto-electric coupling explains circular dichroism signals.

Both real and imaginary parts of chirality contribute to measurable signals.

Proposed scheme allows unambiguous determination of unknown chirality.

Abstract

Metasurfaces, the two-dimensional analogues of metamaterials, are ideal platforms for sensing molecular chirality at the nanoscale, e.g. of inclusions of natural optically active molecules, as they offer large accessible areas (they are essentially surfaces) and can accommodate the necessary strong resonances for coupling the probing radiation with the chiral inclusions. Here, we examine theoretically achiral isotropic metasurfaces, and we treat them as polarizable surfaces that support resonant electric and magnetic currents, which are coupled via the chiral inclusions. We derive analytically, and verify numerically, expressions that provide insight to the enhancement mechanism of the magneto-electric coupling and explain why circular dichroism signals (difference in absorption between left- and right- circularly polarized waves) can arise from both the real and the imaginary part of the chirality parameter $\kappa$. Our analysis demonstrates distinct chiroptical signals where the contributions from both the real and imaginary part of $\kappa$ can be independently observed and, based on such measurements, we propose a scheme for the unambiguous determination of an unknown chirality.